The present invention generally relates to aircraft braking systems and more particularly pertains to improvements in taxi brake inhibit systems that reduce carbon brake wear.
Carbon brakes are fitted to many modern aircraft that are designed to carry large passenger or cargo payloads. Such brakes rely on the use of a carbon composite material to serve as friction material as well a heat sink. A stack of carbon rotor disks and carbon stator disks are coaxially arranged in an alternating sequence along a wheel's axis wherein the rotor disks are rotationally keyed to the wheel while the stator disks are keyed to the stationary axle. Braking force is generated by the pressurization of piston actuators that are configured to compress the stack between a pressure plate and a backing plate to thereby cause the friction surfaces of adjacent disks to engage one another. While carbon brakes are preferred for weight and performance reasons over steel brakes, the cost of replacing the stack as a function of landing cycles between replacements is much higher than for steel brakes.
In contrast to conventional steel brakes for which brake life is largely determined by the total amount of energy that is absorbed, carbon brakes wear as a function of the number of times the brakes are applied, since wear is highest upon initial brake application when the brake temperature is low. Consequently, most carbon brake wear tends to occur during taxiing as the brakes may be applied routinely dozens of times in negotiating the taxiways between the runway and the gate and during the stop-and-go traffic that may be encountered in the queue for take off.
Braking systems have been devised to reduce the number of brake applications and hence the wear rate of carbon brakes by disabling one or more brakes during low energy brake applications, i.e. during taxiing. As such, individual brakes are subject to a lower number of brake applications while the increased braking load during each application has no adverse effect on wear. Such systems may be relied upon to determine the sequence of brake disablements so as to achieve an even wear rate among the various braked wheels without compromising stopping ability and without adversely affecting the stability of the aircraft. Nonetheless, such systems may suffer from certain shortcomings, including harsh and objectionable changes in deceleration rate (“deceleration bumps”), undesirable yaw forces that require countersteer, and changes in brake “feel” when taxi brake inhibit thresholds are exceeded.
Referring to FIG. 1, in the current Boeing 787 aircraft taxi brake inhibit system, without taxi brake inhibit in operation, all taxi brakes operate, and taxi brake energy or brake force (FB) distributes simultaneously to all of the aircraft brakes of the landing gear, resulting in normal brake pedal “feel.” However, as is illustrated in FIG. 2, during operation of conventional Boeing 787 aircraft taxi brake system with a conventional taxi brake inhibit mode in operation, in a first brake configuration (1), a first half of the brakes operate with taxi brake energy or brake force (FB), and a second half of the brakes operate with zero taxi brake energy or brake force (FB), and in a second brake configuration (2), the second half of the brakes operate with taxi brake energy or brake force (FB), and the first half of the brakes operate with zero taxi brake energy or brake force (FB), and the brakes alternate in this manner at each normal taxi brake application to improve carbon brake life, so that twice the torque is applied half the time and the taxi brake energy or brake force (FB) is thereby evenly distributed to all brakes.
However, as is illustrated in FIGS. 3 and 4, depicting brake force (FB) vs. corresponding brake pedal displacement (XP) for operation of aircraft brakes without and with conventional implementation of a taxi brake inhibit mode, application of half the brakes during normal taxi brake operation is typically implemented according to a specific curve of brake force command (FB) vs. brake pedal application (XP), and results in a brake pedal “feel” that provides only half the airplane deceleration per unit pedal force when taxi brake inhibit is active. This in turn results in doubling the change in brake pedal “feel” between taxi brake inhibit “ON” and “OFF”, and doubling of an asymmetric difference in brake pedal “feel” when taxi brake inhibit is “ON” on one side of the aircraft and “OFF” on the other. In addition, as is illustrated in FIG. 7, showing graphs of pedal displacement (XP) vs. elapsed time (t), and brake force (FB) vs. elapsed time (t), because the taxi brake inhibit feature incorporates a tunable 45% brake force command threshold above which the inhibited brakes are applied, a doubling of the deceleration “bump” results each time the “45% Threshold” is exceeded. These changes in “feel” can be very large and objectionable.
For any taxi brake inhibit implementation it is necessary to set a brake force command threshold above which taxi brake inhibit is shut off. This ensures that all brakes operate during an emergency stop. In setting that threshold it must be set high enough so that the threshold is not exceeded during in-service taxi braking, because a very harsh and objectionable “deceleration bump” occurs when the inhibited brakes suddenly become active. However it must also be set low enough so that skids don't occur during in-service taxi braking due to the brake force being doubly high on the “active” brakes to compensate for the other brakes that are “inhibited”, because these skids would also cause harsh and objectionable “deceleration bumps”. Certain taxi brake inhibit features only inhibited ⅓ of the brakes at one time, so a threshold could be set that met both criteria—it was high enough to prevent exceeding the threshold during normal taxi braking, and yet low enough to prevent antiskid activity. Another taxi brake inhibit feature inhibits half the brakes at a time, and a threshold level cannot be set that meets both criteria. As a result the taxi brake inhibit feature will cause harsh and objectionable deceleration bumps during normal taxi braking, either due to the idle brakes suddenly applying (threshold too high), or due to antiskid activity (threshold too low) or both.
Certain aircraft incorporate two brake system control units, one for the brakes on the right side of the aircraft and the other for the brakes on the left side, and neither brake system control unit (BSCU) knows what the brake pedal application, Brake Deactivated status, or Antiskid Fault status are for the brakes on the other side. This creates disadvantages for the taxi brake inhibit feature because it allows one side of the aircraft to have taxi brake inhibit operative while the other side is not. During times when taxi brake inhibit is only operative on one side of the aircraft there will be a very significant difference in brake “feel” between the two sides of the aircraft, which will cause the aircraft to pull to one side during taxi braking (toward the side where all brakes are active). This will be very objectionable to the pilot and may also result in significant energy imbalance between the two sides of the aircraft as the pilot tries to compensate with nose steering.
When the taxi brake inhibit is operating the pilot feels a factor-of-two reduction in brake force for a given brake pedal application. This will cause a pronounced “mushy” feeling in the brake pedals. The pedals must therefore be applied twice as hard for every taxi brake application, and prior experience indicates that this will be objectionable to the pilot. In addition, the difference in pedal “feel” from taxi brake inhibit mode to normal braking mode will very pronounced, which prior experience also indicates will be objectionable to the pilot.
There also a potential problem with the forward pitch-over protection. This is currently handled by a time-based algorithm on the brake force vs. brake pedal application curve, and its implementation needs to be readdressed when incorporating the taxi brake inhibit, because the two features are interrelated. In addition, excessive brake wear can occur during non-normal operation of the taxi brake inhibit feature, such as during deactivation of one brake for up to ten days (“BRAKE DEACTIVATED”), loss of antiskid function to one brake (“ANTISKID FWD” or “ANTISKID AFT”), loss of braking to one brake (“BRAKE FWD” or “BRAKE AFT”), loss of antiskid to both brakes (“ANTISKID STATUS”), or loss of braking to both brakes (“BRAKE CONTROLS”). Two non-normal conditions also commonly result in wheel lockups at 50% normal airplane deceleration. Additional electronic brake failure conditions can also occur that the brake system control unit (BSCU) does not typically monitor that will also result in one or more of the these failure effects. It would be desirable to provide an improved taxi brake system for the Boeing 787 aircraft that overcomes such shortcomings of existing taxi brake systems. The present invention meets these and other needs.